Interfacial instabilities in fluid films

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Thin fluid films with a free surface overlying a solid material (substrate) occur frequently in a variety of situations. In industrial settings they serve to coat and protect surfaces or facilitate the transfer of mass and heat. In living organisms they perform vital functions as does, for example, the tear film covering the eye or the fluid lining of airways in the lungs. Fluid films are also encountered in the environment, such as groundwater permeating unsaturated fractured rock. Other examples are lava flows and glacial dynamics which are governed by the same physical mechanisms as fluid films with much shorter length scales. Fluid films are subject to interfacial instability, in the sense that, under certain conditions, a flow will develop resulting in non-uniformity in the thickness of the film. In time, interfacial instability can lead to the rupture of the film, and if the flow is sufficiently fast, large amplitude waves can form and propagate along the surface. The appearance of interfacial instability often has an adverse effect on the intended function of the fluid film. In coating applications connected to manufacturing processes, for example, a nonuniform film flow can result in an irregular distribution of the coating material. Similarly, fluid films serving a biological function can be ineffective if they become too thin or develop holes. In certain industrial sectors, however, interfacial instability can optimize the process. For example, in heat and mass exchangers the formation of interfacial waves improves the operation of the device due to the fact that there is an increase in the surface area of the liquid-gas interface, which facilitates the transport. A mathematical fluid flow model can be used to anticipate the conditions for the onset of instability and predict its evolution. I propose to implement theoretical models that will investigate the combined effect of various factors affecting the stability of a fluid film that are relevant in real-world situations. Notable factors that will be considered include thermal effects, the presence of surfactants (chemical solutes affecting surface tension), the permeation of fluid through a porous substrate, as well as substrates with nonplanar surfaces exhibiting undulations or corrugations.